A variable frequency drive controls the speed and torque of an alternating current (AC) motor by varying the input frequency and voltage. Three-phase motors provide higher mechanical efficiency, higher power factor and less torque ripple than single-phase motors and are therefore a more desirable choice. Variable frequency drives in the past have generally included a diode rectifier, that converts AC power to direct current (DC) power, connected through a DC bus to an inverter that supplies three phase, variable frequency AC power to a three-phase motor.
When a motor turns faster than the speed designated by the variable frequency drive, the motor acts as a generator, generating power that is returned to the DC bus. In a variable frequency drive with a diode rectifier, the rectification of the AC power to the DC bus is a one-way street and the generated power causes the voltage on the DC bus to rise.
One known method of handling the generated power is to add a dynamic braking resistor to the variable frequency drive. When the voltage on the DC bus rises due to the generated power, the generated power is shunted to the dynamic braking resistor that converts the generated power to heat. Dynamic braking resistors add complexity and expense to a variable frequency drive installation.
The generated power can alternatively be handled with a regenerative variable frequency drive that has an active converter instead of the one-way diode rectifier. An active converter allows power to flow from the AC source to the DC bus and from the DC bus back to the AC source. A regenerative variable frequency drive puts the generated power back onto the line, and thereby reduces the total power consumption of the load.
Regenerative variable frequency drives for three-phase input and output are known. A conventional diode rectifier drive can convert AC power from a single-phase source to charge the DC bus. The known three-phase active converters cannot convert the power from a single-phase AC source to charge the DC bus.
Three-phase AC power is generally supplied to industrial areas. However, only single phase AC power is available to most residential and rural areas. The single phase AC power available in most residential and rural areas is provided by a step down transformer connected to a high voltage line and, in the United States, is normally supplied as about 240 volts at 60 Hz between the first and second input lines. Many three-phase induction motors are operated at high voltage such as about 460 volts to reduce the current passing between the inverter of the variable frequency drive and the motor, and thereby reducing the required size of the connecting cables. Diode rectifier converters cannot directly boost the incoming 240 volts to 460 volts.
Diode rectifiers distort the current drawn from the power grid. This distortion creates harmonic distortions that may affect other users on the grid. The distortion also reduces the power factor. A variable frequency drive with a diode rectifier therefore requires additional circuitry for power factor correction and harmonic filtering.
Some of the more common loads on phase converting variable frequency drives are water pumps, HVAC systems and oil pump jacks. Harmonic distortion of the input current caused by phase converting variable frequency drives is much higher than variable frequency drives with three-phase input because of the large 3rd harmonic associated with the single-phase input. Single-phase passive filters that meet IEEE-519 standards are expensive and not readily available. Single-phase passive filters also have disadvantages such as resonance with the utility, large size and weight, and possible unsuitability for variable loads. In addition to these limitations, passive filters must be designed for particular frequencies and thus may not affect all of the harmonic currents.
One method to reduce input harmonics from phase converting variable frequency drives is the use of an active switching full bridge rectifier. An active switching full bridge rectifier includes power transistors and diodes in the rectifier to actively control the waveform of the input current. An active switching full bridge rectifier improves the input power factor by controlling the input current waveform distortion and is regenerative, allowing power to flow both directions, in and out of the drive. U.S. Pat. No. 8,941,340 to Meiners et al. discloses a regenerative variable frequency drive including an active single phase converter connected to a three phase inverter.
Different control methods have been used to control a single-phase converter with an active switching full bridge. The main control methods for single-phase converters with an active switching full bridge are unipolar voltage switching (UVS) and bipolar voltage switching (BVS). Unipolar voltage switching and bipolar voltage switching each have advantages and disadvantages. The advantages of a unipolar voltage switching scheme are lower switching loss, lower harmonics and an effective switching frequency that is double the actual switching frequency. The advantages of bipolar voltage switching scheme are effectively controlling the current at zero crossing and less ripple current. Hybrid control for Current Source Inverters (CSI) has been proposed to avail the advantages of both unipolar voltage switching and bipolar voltage switching.